What Is the Transmission Coefficient for a Rectangular Barrier?

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Question:

Determine the transmission coefficient for a rectangular barrier. Treat separately the three cases E<0, E=Vo, E>Vo (note that the wave function inside the barrier is different in the three cases.
and
V(x) = Vo > 0 for -a < x < a
= 0 for |x| > a



I attempted to do this like the finite square well and do it per "zone", because I'm not sure how to go about dealing with the energy first.

If I call - infinity -> -a "zone 1" and the potential there is zero, the general solution should be:
\varphi = Ae^{-kx} + Be^{kx}, but the first term blows up to -infinity, so we're left with
\varphi = Be^{kx}.

Can someone tell me what is wrong with this logic?
For E<Vo and x < -a \varphi should include both A and B and be imaginary, but I don't know why.
 
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I'm sure it's apparent that this is from a textbook, (Griffiths 2.33) but it is study, not homework. Please don't disregard on that basis.
 
To generalize a little bit more, this is is my basic question... why is the incoming wave to a square barrier complex?
 
Nevermind.
 
To solve this, I first used the units to work out that a= m* a/m, i.e. t=z/λ. This would allow you to determine the time duration within an interval section by section and then add this to the previous ones to obtain the age of the respective layer. However, this would require a constant thickness per year for each interval. However, since this is most likely not the case, my next consideration was that the age must be the integral of a 1/λ(z) function, which I cannot model.

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